An RF power amplifier provides the final stage of amplification for a communication signal that has been modulated and converted into an RF signal. Often that RF signal exhibits frequencies in a predetermined frequency band that is licensed by a regulatory agency for a particular use. The RF power amplifier boosts the power of this RF communication signal to a level sufficient so that the signal, when it propagates to an antenna, will be broadcast in such a manner that it will meet the communication goals of the RF transmitter.
Many popular modern modulation techniques, such as CDMA, QAM, OFDM, and the like, require the RF power amplifier to perform a linear amplification operation. In other words, the RF communication signal conveys both amplitude and phase information, and the RF power amplifier should faithfully reproduce both the amplitude and phase content of the RF signal presented to it. While perfect linearity is a goal for any linear RF power amplifier, all linear RF power amplifiers invariably fail to meet it. The degree to which the goal of perfect linearity is missed leads to unwanted intermodulation, nonlinearities, and spectral regrowth.
The regulatory agencies that license RF spectrum for use by RF transmitters define spectral masks with which transmitters should comply. The spectral masks set forth how much RF energy may be transmitted from the RF transmitters in specified frequency bands of the electromagnetic spectrum. As transmitter technology has advanced, and as increasing demands have been placed on the scarce resource of the RF electromagnetic spectrum by the public, the spectral masks have become increasingly strict. In other words, very little energy outside of an assigned frequency band is permitted to be transmitted from an RF transmitter. Accordingly, unless the spectral regrowth that results from any nonlinearity in the amplification process is held to a very low level, the RF transmitter may be in violation of its regulatory spectral mask. Spectral masks for lower power, unlicensed bands of the spectrum tend to be more lenient than for higher power, licensed-band RF transmitter applications.
In addition to linearity requirements set through spectral masks, power-added efficiency (PAE) is another parameter of interest to those who design RF transmitters. PAE is the ratio of the RF output power to the sum of the input RF power and the applied bias-signal power. An amplifier that has low PAE wastes power, which is undesirable in any transmitter, but particularly undesirable in battery-powered transmitters because it necessitates the use of undesirably large batteries and/or undesirably frequent recharges. Conventionally, improvements in PAE have been achieved at the expense of linearity. But envelopes tracking (ET) techniques, envelope elimination and restoration (EER) techniques, and hybrids between the two techniques have shown promise for achieving PAE improvements. When such techniques are combined with conventional digital predistortion techniques, the RF power amplifiers may also achieve modest amounts of linearity.
Envelope tracking (ET) refers to a general technique for biasing an RF power amplifier using a time-varying signal that at least roughly tracks the envelope of the RF communication signal. In accordance with ET techniques, a full RF communication signal exhibiting both amplitude and phase modulation is supplied to an input of the RF power amplifier, and the RF power amplifier is configured to perform substantially linear amplification. When the amplifying device at the core of the RF power amplifier is operated exclusively in its saturation region, its gain is less sensitive to fluctuations across its drain/collector and source/emitter terminals. When a bias signal applied to this amplifying device tracks the envelope and is maintained at a level no greater than it needs to be to maintain operation in the device's saturation region, the device generally operates as a somewhat linear amplifier and wastes less power than it would using a constant bias signal.
Envelope elimination and restoration (EER) may be viewed as a specialized form of ET. In accordance with EER, a biasing signal applied to the amplifying device at the core of the RF power amplifier should identically reproduce the envelope of the RF communication signal. Since this biasing signal essentially is the envelope, the envelope may be eliminated from the RF communication signal before an RF signal is applied to the input of the RF power amplifier. This applied RF signal is only angle or polar modulated and not amplitude modulated. Consequently, the core amplifying device may be operated as an inherently more efficient switching amplifier. Amplitude modulation is restored to the angle or polar modulated RF signal at the output of the core amplifying device through the envelope tracking biasing signal.
Hybrid techniques represent a combination of the more generic ET and more specific EER techniques. In particular, the core amplifying device of the RF power amplifier is typically configured for substantially linear operation, and the core device is biased using a signal that closely tracks the envelope of the RF communication signal. Such hybrid techniques can achieve substantial improvements in PAE compared to other linear amplifiers, but in order to maintain modest amounts of linearity, other compensating techniques in additional to conventional digital predistortion are required.
In particular, using field effect transistor (FET) amplifier nomenclature, in order to minimize amplifier nonlinearity it is desirable to operate a substantially linear amplifier so that VDS≧VGS−VT, where VDS is the drain-source voltage, VGS is the gate-source voltage, and VT is a threshold associated with a gate-to-source voltage below which the drain-to-source current is zero. The threshold voltage VT is roughly the VGS voltage where the core amplifying device begins to conduct. By maintaining VDS≧VGS−VT, the core amplifying device remains in its saturation region where substantially linear amplification may result. Due to filtering circuits which typically surround the core amplifying device, the RF power amplifier may experience brief instants less than the duration of a complete cycle of the RF waveform where VDS<VGS−VT. But prolonged durations greater than a complete cycle of the RF waveform with VDS<VGS−VT lead to significant nonlinearity in the operation of the RF power amplifier and to spectral mask violations.
Conventional digital predistortion techniques may compensate for some nonlinearities that result from signal anomalies occurring on a baseband or video band time scale. Conventional digital predistortion techniques apply a predistorting gain to the digital baseband communication signal upstream of amplification to compensate for amplifier gain fluctuations that are experienced over a duration of many RF waveform cycles. This duration reflects the baseband or video band time scale. When an RF power amplifier operates in a VDS<VGS−VT condition for an extended period of time, the amplifier is likely to be applying a gain of zero and/or a gain that is highly dependent upon instantaneous values of VDS for the extended period of time. Conventional digital predistortion techniques are unable to adequately compensate for such gain fluctuations. For example, when the amplifier applies a gain of zero, the predistortion would need to provide an infinite gain to compensate. The RF power amplifier will apply a gain of zero whenever VGS<VT. Such operation is referred to as gain collapse. Providing infinite predistortion gain to compensate for gain collapse is not achievable using realistic components.
When an RF power amplifier is being operated as an ET, EER, or hybrid amplifier, a biasing signal that closely tracks the RF communication signal envelope may occasionally exhibit very low levels in the baseband or video signal time scale, even corresponding to where VGS<VT, because the envelope occasionally exhibits such low levels. In accordance with the field effect transistor (FET) example, when this happens, an ET biasing signal that leads to high levels of PAE may cause VDS to exhibit very low levels, and the RF power amplifier operates in the VDS<VGS−VT condition for an extended period of time. Moreover, conventional digital predistortion techniques are powerless to compensate for the resulting significant nonlinearity and spectral regrowth that occur. Consequently, RF transmitters with RF power amplifiers operated as ET, EER, or hybrid amplifiers employ waveform shaping in addition to digital predistortion to prevent operation in the VDS<VGS−VT condition and to promote operation in the VDS≧VGS−VT condition for all extended periods of time. Such waveform shaping techniques are also known as detroughing because the troughs of the envelope signal are boosted or filled prior to being used to bias the RF power amplifier at levels that promote operation in the VDS≧VGS−VT condition.
Conventional detroughing techniques utilize an instantaneous mapping process performed at baseband to detrough the envelope signal. Typically, a magnitude signal is formed from a communication signal. The magnitude signal describes the envelope of the RF communication signal to be amplified. When the magnitude level over a period equivalent to many RF waveform cycles is low, the magnitude is boosted. The minimum period over which the envelope waveform is shaped is determined by a sampling rate for digital circuits which perform predistortion and waveform shaping. In one example, the detroughed magnitude is instantly boosted to a predetermined level when the magnitude signal is below a threshold, and the detroughed magnitude otherwise equals the magnitude signal. For this example, a small amount of amplifier linearity is restored without substantially harming PAE. In other examples, the detroughed magnitude is instantly boosted by a predetermined amount that varies with the precise value of the magnitude signal. Greater amounts of boost are used for lower magnitudes and smaller amounts are used for higher magnitudes. For these examples, somewhat better amplifier linearity is restored without substantially harming PAE.
Unfortunately, the conventional detroughing techniques merely tend to replace greater amounts of nonlinearity resulting from one characteristic of RF power amplifier operation with lesser amounts of nonlinearity resulting from another characteristic of RF power amplifier operation. As a result, conventional RF power amplifiers which employ ET and waveform shaping tend to achieve only modest linearity with significant PAE improvements.
What is needed is an RF transmitter having an RF power amplifier that achieves significant PAE improvements with linearity improved over the linearity achievable using conventional detroughing techniques.